Method of measuring distance between two observation points

ABSTRACT

The present invention relates to a telemeter and to a method of determining the distance separating two points. The method comprises the steps consisting in (a) comparing the phase (φ) of the modulation of an emission signal and of a received returned modulated signal (16), (b) in generating a signal representative of the value of the instantaneous variance (V(φ i )) of the phase samples (φ i ) (107), (c) in generating an estimation of the phase on the basis of a signal derived from the average (φ E ) of n phase samples (φ i ) which have been weighted by their instantaneous variance, (d) in generating a signal representative of the value of the distance (D) on the basis of the estimated phase value (φ E ), and (e) in displaying said value of the distance.

The invention relates to telemeters, i.e. to apparatuses for measuringdistance by means of the go-and-return propagation time of a signalbetween two observation points.

The invention relates more particularly to a method of measuringdistance and to a telemeter implementing the method. They are adapted toturbulance in the propagation medium.

BACKGROUND OF THE INVENTION

FIG. 1 is a diagram of a conventional optical telemeter.

As shown in FIG. 1 many current optoelectronic telemeters use a lightemitting diode (LED) 1 disposed at a first observation point andassociated with a modulator 2 controlled by a high frequency oscillator3 operating at a frequency F_(T) which is preferably several MHz, e.g.15 MHz. The LED is disposed at the focus of a first lens 4 to emit aninfrared light signal which is amplitude modulated at the frequency ofthe high frequency signal generated by the oscillator 3. The lightsignal is in the form of a thin beam which is thus capable ofpropagating a considerable distance.

A trihedral reflector 5 disposed at a second observation point returnsthe beam parallel to itself in a symmetical arrangement whose center isvery close to the vertex of the reflector 5, and the returned beam isfocused by a receive lens 6 adjacent to the above-mentioned send lens 4.

A photodiode is located at the focus of the second lens 6, and ingeneral the photodiode will be of the avalanche type (APD). It feeds asignal to an amplifier stage 8 which delivers an electrical outputsignal which is modulated and phase shifted by an angle φ relative tothe send modulation signal. The angle φ is related to the propagationtime of the signal by the following equation: ##EQU1## in which:

F_(T) is the frequency at which the LED 1 is modulated;

D is the distance between the prism 5 of the telemeter (1, 7) (i.e. halfthe total path followed by the electromagnetic radiation); and

c is the speed of light.

In order to determine the distance separating the two observation pointswith high accuracy (i.e. about 1 mm), it is theoretically necessary tohave very fine measuring resolution (about 3.3×10⁻¹² sec).

For this reason, when the modulating signal is at a high frequency, ithas become conventional to measure the phase shift between two lowfrequency signals.

To begin with, the received modulation signal generated at the outputfrom the amplifier stage 8 is transposed to a low frequency signal(F_(O) -F_(T)) (e.g. about 1,500 Hz) by mixing the received modulationsignal in a mixer 9 with the signal from a local oscillator 10. Itshould be recalled that the low frequency signal obtained at the outputfrom the mixer conveys the same phase information as the received highfrequency modulation signal.

Secondly, the high frequency F_(T) and the high frequency F_(O) from thelocal oscillators 3 and 10 are applied to a second mixer 11 to produce areference low frequency signal.

This low frequency reference signal is applied together with the highfrequency F_(O) signal from the oscillator 10 to a digital phasemeter16, while the low frequency transposed received modulation signal fromthe mixer 9 is also applied to the phasemeter 16, via a low frequencyamplifier 12 an automatic gain control stage 13. a bandpass filter 14for the frequency band Δν₁, and a pulse shaping stage 15 whichtransforms the analog signal from the filter output into a logicalsquare wave signal. These various components are designed so that phasemeasurement is independent of the signal amplitude of the receivedmodulation.

The digital phasemeter 16 determines the amplitude of the phase shift φbetween the emitted signal and the received signal. Phase samples φ_(i)generated by the phasemeter 16 are processed by a processor unit 17which determines the arithmetic mean φ_(A) of the samples: ##EQU2##

The symbol " " is used in the present application to designatedestimated values.

The processor unit 17 then determines the amplitude of the distanceseparating the two observation points on the basis of equation (1).

The signal produced by the processor unit 17 is digitized to display thedistance on a display 18.

It should be observed that to resolve the ambiguity due to each passageof the phase angle φ through 2π, i.e. multiples of the distancec/2F_(T), measurements are conventionally performed at one or morefrequencies whidh are lower than the frequency F_(T). These arrangementsare well known to the person skilled in the art and are not describedfurther.

It should also be observed that the filter 14 conventionally has abandwidth Δν₁ which is relatively narrow (about 100 Hz) in order toreduce detection noise levels before being applied to the input of thephasemeter 16. The phase samples φ_(i) are thus correlated and it is notnecessary to sample at a period of less than kT, where T is the lowfrequency period obtained at the outputs of the mixers, and k is anumter such that kT is less than 1/Δν₁.

In fact, taking the arithmetic mean φ_(A) constitutes a stage of digitalfiltering in addition to the analog filtering.

It should also be observed that with a large number of elementarymeasurements φ_(i) which are highly correlated, the variance of theresult V(φ_(A)) tends towards a limit which does not depend on thenumber of samples, but rather depends only on the measurement time t_(m): ##EQU3## (where kO is a constant).

The Applicant seeks to provide an improved telemetry method whichenables the accuracy of the measurement to be improved and also enablesthe range to be increased.

The Applicant has observed that the medium through which the signalpropagates between the two observation points is generally formed bymasses of air in movement which are not homogeneous in temperature. Thisresults in a field of refractive index gradients n_(i) in which theamplitudes and directions of the field are random in time.

Although the resulting optical path is in fact a sinuous path,calculation and experiment have shown that the increase in path lengthbetween the two observation points is negligible, and thus thatturbulance does not directly affect the phase measurement.

In contrast, turbulance does occasionally cause the signal to disappearthereby erratically reducing the signal to noise ratio and thus reducingthe accuracy of the measurement since: ##EQU4## in which:

V(φ_(i)) represents the variance in the phase samples φ_(i) ;

S represents the received signal of amplitude which is random in time;

N_(A) represents the r.m.s. noise value; and

k₁ is a constant.

The spectrum of the amplitude modulation applied to the signal by theturbulance, herein after referred to as the turbulance spectrum has abandwidth ΔF.

Further, experiments have shown that the amplitude of the signalexpressed in dB is a random variable obeying the normal distributionlaw, i.e. it can be defined by an average μ_(s) and a standard deviationσ_(s).

Attempts could be made to improve the performance of a telemeter in thepresence of turbulance, e.g. by using lenses of large diameter in orderto simultaneously increase the average μ_(s) of the signal and to reducethe standard deviation σ_(s) thereof.

However, such a solution also increases the volume, the weight and theprice of the apparatus. Furthermore, such a solution does notnecessarily improve the accuracy of the final measurement.

Preferred implementations of the present invention provide a new methodof determining the distance between two points by reducing thesensitivity of a telemeter to turbulance without changing the outwardappearance of the telemeter instrument.

SUMMARY OF THE INVENTION

The present invention provides a method comprising the steps of emittingoscillator modulated electromagnetic radiation from a first pointtowards a second point, in reflecting the said electromagnetic radiationfrom the second point back towards the first point, in receiving thereflected electromagnetic radiation at the first point by means of areceiver, in comparing the phase of a first signal derived from thereceiver and of a second signal derived from the emission modulation,and in generating phase samples φ_(i) representative of the phasedifference between the first and second signals, the improvement whereinthe method further includes the following steps:

(i) in generating a signal representative of the instantaneous varianceV(φ_(i)) of the phase samples φ_(I) ;

(ii) in generating as an estimation of the phase an intermediate signalderived from the average φ_(E) of n phase samples φ_(i) weighted bytheir instantaneous variance;

(iii) in generating a useful signal representative of the value of thedistance (D) on the basis of the phase estimation φ_(E) ; and

(iv) in displaying this distance value.

Advantageously the components of the signal detection chain are designedin such a manner that the range of signal amplitude variation passedwithout distortion by the analog signals covers the width of thedistribution function of the received signal amplitudes, i.e. a range offour to six times the standard deviation σ_(s) of the signal.

The equivalent passband Δν₂ of the filters of the signal detectioncircuits is greater than the turbulance spectrum width ΔF.

Preferably, the above-mentioned step (ii) is performed by taking as thevalue representative of the instantaneous variance of the phase samples,a signal which is proportional to the inverse of the square of the valueof the noisy signal under consideration over a passband which isslightly larger than or equal to the turbulance spectrum width.

The advantages provided by the invention are explained below.

In a variant, the above mentioned step (i) is performed by means of adetection chain which takes a set of n_(e) samples at a rate n_(e) timesgreater than the rate at which the n phase samples φ_(i) are taken. Themethod in accordance with the invention advantageously further includesa preliminary step consisting in controlling an optical attenuatorinserted on the propagation path of the electromagnetic radiation insuch a manner than the average μ_(s) of the received signal S is lessthan the upper limit of the signal which can be received withoutdistortion (h) by a factor of k₄ times the standard deviation σ_(s),where k₄ lies between 1 and 2.

Advantageously the method includes the steps of:

(a) measuring the r.m.s. noise value N_(A) ;

(b) generating a signal representative of the average μ_(s) and thestandard deviation σ_(s) of the signal on the basis of a collection ofsamples y_(i) of the signal (in decibels) together with the value of theattenuation A_(j) (in decibels) applied to the radiation; and

(c) generating a signal representative of an estimate of the phasevariance V(φ_(E)) given the r.m.s. noise value, the average μ_(s) andthe standard deviation σ_(s).

Advantageously, the method includes the step of generating a signalrepresentative of an evaluation of the standard deviation of thedistance σ_(D) proportional to the square root of the phase varianceestimation V(φ_(E)).

Preferably the method in accordance with the invention includes afurther step consisting in increasing the gain M of the receiver inincrements until one of the three following conditions is satisfied:

(1) receiver bias is optimized;

(2) the gain M has reached its maximum permitted value; and

(3) the average μ_(s) of the signal has reached an optimum valuecorresponding to the upper limit of the signal which can be admittedwithout distortion (h) as reduced by k₄ times the standard deviationσ_(s), where k₄ lies in the range 1 to 2.

In one particular case, the receiver is an avalanche photodiode and theabove-mentioned condition (1) is obtained when the noise N_(A) underambient lighting which is referred to hereafter as the "ambient noise",is about twice the noise N_(O) from the amplifiers.

Preferably, the generation of the signals representative respectively ofthe estimation of the phase variance V(φ_(E)) and of the standarddeviation of the distance φ_(D) deduced therefrom, together with thedisplay of the standard deviation of the distance is reiterated afteradjusting the gain M of the receiver.

Advantageously, steps, (i) to (iii) are continued until the standarddeviation σ_(D) of the distance is less than a predetermined maximumvalue (σ_(D))_(M) thus increasing the number n of samples taken andimproving the accuracy of the measurement.

The present invention also provides a telemeter comprising:

transmitter means associated with an oscillator suitable for emittingmodulated electromagnetic radiation towards an observation point;

receiver means suitable for detecting the electromagnetic radiationafter reflection at the said observation point;

comparator means suitable for comparing the phase of a first signalderived from the receiver and of a second signal derived from theemitted modulation, in order to generate phase samples φ_(i)representative of the phase difference between the first and secondsignals;

the improvement wherein the telemeter further includes:

means suitable for generating a signal representative of the value ofthe instantaneous variance V(φ_(i)) of the phase samples φ_(i) ;

processor means suitable for generating an intermediate signal derivedfrom the average φ_(E) of n phase samples φ_(i) weighted by the saidvalue representative of their instantaneous variance;

means suitable for generating a useful signal representative of thedistance travelled by the electromagnetic radiation between thetransmitter means and the receiver means on the base of the saidintermediate signal derived from the weighted average φ_(E) ; and

display means responsive to the useful signal and adapted to displaydata representative of the distance travelled by the radiation.

Preferably, the means suitable for generating a signal representative ofthe value of the instantaneous variance comprises means sensitive to thevalue of the noisy signal under consideration in a passband which isslightly wider than or equal to the turbulance spectrum width.

Advantageously, the processor means are adapted to generate a signalrepresentative of the estimated average of the phase φ_(E) on the basisof the equation: ##EQU5## where φ_(i) represents phase samples andY_(i)Δν2 represents the value of the noisy signal from a recursivefilter having bandwidth Δν₂.

In a variant, the telemeter may comprise a first detection chain fortaking the phase samples φ_(i) and a second detection chain suitable fortaking a set of ne samples at a sampling rate which is ne times greaterthan the rate at which the n phase samples φ_(i) are taken, for thepurpose of determining their instantaneous variance V(φ_(i)).

Advantageously the telemeter further comprises:

an optical attenuator comprising a plurality of filters inserted on thepropagation path of the electromagnetic radiation; and

control means responsive to the average μ_(s) of the received signal Sand suitable for controlling the positioning of the attenuator in such amanner than the said average is kept below the upper limit of the signalwhich can admitted without distortion by a factor of k₄ times thestandard deviation σ_(s), where k₄ lies in the range 1 to 2.

Advantageously the telemeter further comprises:

means for generating a signal representative of the r.m.s. value of theambient noise N_(A) ;

means for generating signals representative of the average μ_(s) and ofthe standard deviation σ_(s) of the signal expressed dB;

means suitable for generating a signal representative of the estimatedvariance of the phase on the basis of the equation: ##EQU6## in which k₂and a are constants, tm represents the measurement time, the notation F[] represents the normal law distribution function of the centered andreduced variable, and h designates the upper limit of the signal whichcan be admitted without distortion; and

means sensitive to the said signal and suitable for displaying theestimated phase variance.

Advantageously the telemeter further comprises an optical attenuatorcomprising a plurality of filters inserted on the propagation path ofthe electromagnetic radiation, and the means suitable for generatingsignals representative of the average μ_(s) and the standard deviationσ_(s) of the signal expressed in dB are adapted to generate signalswhich correspond respectively to: ##EQU7## in which equation y_(i)represents the signal samples expressed in dB and A_(j) represents theattenuation of the signal due to the attenuator as associated withrespective samples y_(i), while p represents the total number of samplestaken into consideration. It should be observed that Y_(i) is usedherein to represent signal samples which are linear.

Preferably, a telemeter in accordance with the invention furthercomprises:

means suitable for generating a signal representative of an evaluationof the standard deviation of the distance σ_(D) proportional to thesquare route of the estimation of the phase variance V(φ_(E)), orV(φ_(E)); and

means suitable for displaying the standard deviation of the distanceσ_(D).

Advantageously the telemeter in accordance with the invention includesmeans suitable for controlling incremental increases in the gain M ofthe receiver until one of the three following conditions is satisfied:

(1) the receiver bias is optimized;

(2) the gain M has reached its maximum permitted value; and

(3) the average of the signal μ_(s) has reached an optimum valuecorresponding to the upper limit of the signal admitted withoutdistortion (h) as reduced by a factor of k₄ times the standard deviationσ_(s), where k₄ lies in the range 1 to 2.

In a particular case, the receiver is an avalanche photodiode and theabove-mentioned condition, (1) is obtained when the ambient noise N_(A)is about twice the amplifier noise N_(O).

Advantageously, the telemeter includes means suitable for comparing thestandard deviation σ_(D) with a predetermined maximum value to controlthe accumulation of samples and to improve the accuracy of themeasurement for as long as the standard deviation is greater than thesaid predetermined value.

BRIEF DESCRIPTION OF THE DRAWINGS

An implementation and an embodiment of the invention are described byway of example with reference to the accompanying drawing, in which:

FIG. 1 is a block diagram of the prior art and has already beendescribed;

FIG. 2 is a block diagram of a telemeter in accordance with theinvention; and

FIGS. 3A, 3B and 3C should be taken together as a single flowchartshowing the steps in the performance of the method in accordance withthe invention.

MORE DETAILED DESCRIPTION

We begin by describing the method of determining the distance D betweenthe two observation points under consideration by means of the presentinvention.

As mentioned above, the distance D is determined on the basis of thephase shift φ by means of a relationship based on equation (1).

However, unlike conventional telemeters in which the best estimation ofthe phase was taken as being constituted by the arithmetic mean φ_(A) ofn phase samples φ_(i), ##EQU8## the present invention now proposes toevaluate the distance D on the basis of a phase estimate determined asthe average φ_(E) of n samples φ₁ weighted by their instantaneousvariance V(φ_(i)) such that: ##EQU9##

The improvement provided by such processing in accordance with theinvention may be evaluated by calculating the ratio of the variancesV(φ_(A)) and V(φ_(E)) of the arithmetic mean φ_(A) and of the weightedmean φ_(E).

The variance of φ_(A) and of φ_(E) is not constant from one group ofmeasurements to another. However, if the time during which samples aretaken t_(m) is long enough it can be shown that V(φ_(A)) and V(φ_(E))tend respectively to: ##EQU10## in which equations: ##EQU11##

k₂ is a constant,

b represents the minimum threshold for the admitted signal expressed indB;

h represents the upper limit of the signal expressed in dB that can beadmitted without distortion,

N_(A) represents the r.m.s. value of the ambient noise N_(A),

μ_(s) represents the average signal amplitude;

σ_(s) represents the standard deviation of the signal; and

the notation F [ ] represents the normal law distribution function ofthe reduced centered variable.

More precisely, the above-mentioned evaluation of the improvementprovided by the invention may be determined by calculating efficiency ηfor different values of standard deviation σ_(s) ##EQU12## as a functionof the average μ_(s).

Thus, on the basis of (6) and (10) we obtain: ##EQU13##

The main difficulty in implementing the above-mentioned method and moreparticularly for determining φ_(E) on the basis of equation (9) lies inacquiring the instantaneous variance V(φ_(i)) to perform the weighting.

In theory, the estimate of V(φ_(i)) should be obtained from a set of nesamples taken at a sampling rate ne times the sampling rate used forφ_(i), (i.e. (ne/kT).

To do this, with phase samples φ_(i) being taken at a rate of (1/kT), asecond low frequency detection chain may be provided operating at thefrequency (ne/kT), and having a passband of (neΔν₂ /k) so that thecorrelation between the measurements does not increase the uncertaintyin the estimate V(φ_(i)).

The term second detection chain is used to designate either an assemblycomprising a mixer, an amplifier, an AGC stage, a filter, a pulseshaping circuit, and a phase meter connected to the output of theamplifier 8, or else simply a phase meter providing phase samples whichare separated by a phase increment of (2πne/k).

However, the Applicant proposes a method avoiding the requirement toprovide two detection chains.

To do this, the Applicant has observed, firstly that the r.m.s. noisevalue N_(A) may be taken to be constant over the few seconds that ameasurement requires.

Thus:

    N.sub.A.sup.2 =N.sub.O.sup.2 +2qη.sub.q P.sub.s M.sup.α Δν.sub.2,                                        (13),

in which

N_(O) represents the amplifier noise;

q=1.6×10⁻¹⁹ c

M represents the multiplication factor with a photodiode 7;

α represents the ionization coefficient which is generally about 2.3;

η_(q) represents the quantum efficiency and is generally about 0.3;

P_(s) represents the optical power received by the photodiode; and

Δν₂ represents the equivalent passband of the detection circuit filters.

The coefficient M is kept constant by the control circuits during anyone measurement, the factor P_(s) is thus the only factor likely to varyN_(A).

However, the field detected by the receiver optics extends only over afew minutes (i.e. a few meters at 1 km). Thus, the probability of theillumination of the "observed" zone changing, i.e. the probability ofambient sunlight changing, during the short time required for ameasurement is very low.

Further, although the signal S does not exist on its own, the filteringperformed by the detector circuits reduce the noise to a negligiblelevel relative to the signal, so it is possible to write:

    Y.sub.iΔν.sbsb.2 ≅S                     (14)

in which Y_(i)Δν.sbsb.2 represents the value of noisy signal coming froma recursive filter having a pass bandwidth of Δν₂.

Consequently, the Applicant has determined that during a measurementequation (4) reduces to

    V(φ.sub.i)=k.sub.1 N.sub.A.sup.2 /Y.sub.i.sup.2.sub.Δν.sbsb.2( 15)

By combining equations (9) and (15) we obtain: ##EQU14##

The phase estimation φ_(E) as determined on the basis of equation (5)and thus the determination of the distance D is therefore simplified andmade possible using a conventional telemeter.

A telemeter in accordance with the invention is now described in greaterdetail with reference to FIGS. 2, 3A, 3B and 3C.

As shown in FIG. 2, the processor unit UC in accordance with theinvention is sensitive firstly to the output signal from the phasemeter16 representative of the phase shift φ_(i), and secondly to a signaltaken from the output of the amplifier 12 and representative either ofthe noisy signal or else of the noise alone, depending on the on or offstate of the emission LED 1.

It may be observed that this LED is connected to the modulator 2 via aswitch 119 under the control of a gate 123.

When the signal applied to the gate 123 closes the switch 119, the LED 1is powered and the measuring chain is sensitive to the noisy signal.

Conversely, when the signal applied to the gate 123 opens the switch119, the measurement chain is sensitive to noise only since the LED isunpowered.

The second signal representative of the noisy signal or of the noise onits own to which the central processor unit UC is sensitive is appliedthereto via a series assembly comprising firstly a sampling stage 100connected to the output of the amplifier 12 for detecting the peaks ofthe sinusoidal signal appearing at the output thereof, and secondly ananalog-to-digital converter 101.

Further, for a telemeter utilization range running from a few meters toseveral kilometers, the dynamic range of optical power received isgreater than 10⁶ (i.e. 120 dB) of signal strength.

For this reason, as shown diagrammatically in FIG. 2, the telemeterincludes a variable optical attenuator inserted on the path of theelectromagnetic radiation and suitable for varying the optical power ofthe signal in steps to avoid the electronic circuits from beingsaturated by a signal of amplitude greater than h.

More precisely, the optical attenuator comprises a set of gray filters120 which are rotatable in front of the receiver diode 7 under thecontrol of a motor 121. The position of the filters 120 is monitored bya coder 122.

The structure of the central unit UC shown diagrammatically in FIG. 2 isdescribed with reference to the flowchart of FIGS. 3A to 3C. As shown inFIG. 3A, the process of measuring the distance D is initialized by anoperator acting on the telemeter keyboard (step 200 KEY IN DISTANCEMEASURING COMMAND).

After this instruction, the central processor unit UC of the telemeterreceives initially by measuring the noise in ambient light. As explainedbelow, this noise measurement is used for evaluating the standarddeviation σ_(D) of the distance D.

In FIG. 2. the noise measuring process is diagrammatically illustratedby the link between the converter 101 and a block 114 via a switch 102.The block 114 performs the function ##EQU15##

To perform this measurement, at step 201 (LED=0 & A_(j) =0) the centralcontrol unit firstly opens the switch 119 and thus turns off the LED 1,and secondly positions the gray filters 120 to obtain minimum signalattenuation.

At the following step 202, the central unit determines the r.m.s. valueN_(A) of the ambient noise on the basis of the equation ##EQU16## inwhich X_(i) represents the samples of ambient noise.

In order to acquire the non-correlated noise samples X_(i) rapidly, theyare taken in the band Δf at each period T.

The value of m (the number of samples) is determined according to acompromise between the desired accuracy for the variance V(φ_(E)) and anacceptable acquisition time for the samples X_(i).

The squares of the samples, X_(i) ², are summed by means referencediagrammatically as 114 in FIG. 2.

The average μ_(s) and the standard deviation σ_(s) of the amplitudes ofthe signal y_(i) are then evaluated in dB.

In FIG. 2 this process is illustrated diagrammatically by establishing alink between the converter 101 and the block 103 via the switch 102. Theblock 103 serves to reject samples y_(i) >h.

To measure μ_(s) and σ_(s) correctly, the value h must not be exceededsince that would create amplitude distortions.

When the time taken to perform the entire measuring program is notcritical, it may begin by accumulating voltage samples after a period ofa few seconds during which any overshoot by the signal causes theoptical attenuation ΔA to be incremented by actuating the filter 120.

The following values are then obtained: ##EQU17##

However, when the time taken for complete program operation is to beminimized, the control of the filter 121 may be speeded up by amonitoring device 117 which receives information representative of thesignal overshooting h and representative of the position of the filter.For example, if the filter 120 is a rotary disk having a progressivegray scale, an incremental coder may supply a number correspsonding tothe attentuation A_(j).

Each time the threshold h is exceeded, the disk turns through jincrements corresponding to an attenuation ΔA. During time intervals forwhich y_(i) is less than h two sums are performed: ##EQU18##

Further, the attentuation marked ΔA must be chosen in such a manner thatthe maximum number of high amplitude measurements suppressed bycomparison with the threshold h is small compared with p.

When this condition is satisfied, equations 17 and 18 become ##EQU19##

In order to do this as shown in FIG. 3A at step 203 (LED=1 & A_(j) =0)the LED 1 is excited by closing the switch 119, with the signalattenuation caused by the filters 120 still being at a minimum value.

At the following step 204 (y_(i) >h) (performed by the means referenceddiagrammatically as 103 in FIG. 2) a test is performed to see whetherthe value y_(i) of the noisy signal is greater than or less than a valueh corresponding to the upper limit of signal which can be admittedwithout distortion.

If the upper limit h is exceeded, step 204 is followed by a step 205(A_(j) =A_(j-i) +ΔA) during which the motor 121 is powered from themonitoring block 117 to move the gray filters 120 to increase theoptical attenuation of the signals by one step.

After each increment in attenuation, the test of step 204 is repeated.Signals y_(i) of amplitude less than the limit h are thus accumulated inassociation with attenuation values A_(j), and after passing through arecursive filter Δν₂ (104) the noisy signals y_(i) are compared with apredetermined threshold b at step 206.

If the value of the noisy signal is less than the threshold b, an alarmis triggered at step 207 to warn the operator that the signal levelbeing received is too low for proper measurement.

In contrast, if the value of the noisy signal is greater than thethreshold b, step 206 is followed by steps 208 and 209.

In step 208 the means 113 of the central unit determine a signalrepresentative of the value ##EQU20## while in step 209 the means 112 ofthe central unit determine a signal representative of the value##EQU21## in which relationships A_(j) represents the amplitude of theattenuation inserted by the filters 120.

In FIG. 2 this process is illustrated diagrammatically by establishing alink between block 105 and blocks 112 and 113 via switch 106.

Steps 209 and 209 are then followed by a step 210 during which the meansreferenced 115 in FIG. 2 which are connected to blocks 112 and 113determine firstly the average μ_(s) and secondly the standard deviationσ_(s) on the basis of equations

    μ.sub.s =W.sub.p /p,                                    (21)

and

    σ.sub.s.sup.2 =Z.sub.p /p-μ.sub.s.sup.2           (22)

which are derived respectively from equations (7) and (8) and in which prepresents the number of samples taken into account during steps 208 and209.

At the following step 211 the central unit UC proceeds to estimate thevariance of the phase φ_(E) on the basis of equation ##EQU22##

The means referenced 116 in FIG. 2 then proceed at step 212 to perform apreliminary estimate and to display standard deviation of the distanceσ_(1D) by generating a signal representative of: ##EQU23##

As a function of the value displayed, the operator decides whether ornot to continue with the measurement.

If necessary, the operator may modify the measurement period t_(m) orthe parameters of the optical link (e.g. the number of prisms).

Assuming that measurement does continue, step 212 is followed by a teststep 213 (FIG. 3B) during which the estimated value σ_(1D) of thestandard deviation is compared with a predetermined maximum value(σ_(D))_(M) for the standard deviation.

If the estimated value σ_(1D) is less than the maximum value (σ_(D))_(M)step 213 is followed by step 214 during which the means 117 of thetelemeter adjust the position of the gray filters 120 so that theaverage μ_(s) obeys the equation

    μ.sub.s =h-k.sub.4 σ.sub.s                        (24)

in which h represents the upper limit of the signal which can beadmitted without distortion and k₄ is a factor in the range 1 to 2 whichdepends on the average duration of the periods for which the threshold hwas exceeded and on the settling time allowed for the circuits to comeback to their range of linear operation.

Step 214 is followed by step 226.

In contrast, if the estimated value of the standard deviation σ_(1D) isgreater than the maximum value (σ_(D))_(M), step 213 leads to anadjustment of the gain M of the avalanche photodiode 7 under the controlof the means 118 of the central unit UC.

The decision to lengthen the measurement program to increase M, which isset to a minimum value M_(O) when the system is initialized is thusdetermined as a function of the value of the standard deviation.

To adjust M to its optimum value when σ_(1D) >(σ_(D))_(M), a measurementis initially made of the dark noise.

The central unit UC controls the switch 102 (FIG. 2) to connect theconverter 101 to the means 114.

Furthermore, at step 215 (LED=0 & A_(j) =A_(M)) the LED 1 is turned offby opening the switch 119 and the filters 120 are activated bycontrolling the motor 121 to cause maximum attenuation A_(M) of thesignal.

Then at step 216 the value of the noise due to the amplifiers isdetermined by means of the equation: ##EQU24##

Then at step 217 the motor 121 is again activated so that the filters120 cause minimum attenuation of the optical signal, while the LED 1 isleft switched off, thereby measuring the ambient noise N_(A).

Step 217 is followed by step 219 during which the central unit testswhether the ratio N_(A) /N_(O) is greater than 2.

If it is greater than 2, the bias of the avalanche photodiode isoptimized and it is not necessary to increase M so step 219 is followedby the above-mentioned step 226.

If the ratio is not greater than 2, the photodiode bias is not optimizedso step 219 is followed by step 220 (M_(j) =M_(j-1) +ΔM) during whichthe gain M is increased by increments ΔM, where M and ΔM are expressedin dB.

At the following step 221 the r.m.s. value of the ambient noise N_(A) isdetermined in a similar manner to the above-mentioned step 202 by meansof the equation: ##EQU25##

Finally, at step 223 the central unit determines the value of theaverage μ_(s) using the equation (26)

    μ.sub.sj =M.sub.j -M.sub.O +μ.sub.sj-1               (26)

At step 224 the central unit tests whether the factor M_(j) has reachedthe maximum permitted value M_(M).

If so step 224 is followed by step 226.

Otherwise M is less than M_(M) and the central unit performs 225 to testwhether the signal average μ_(sj) has reached the optimum value (32)μ_(sj) =h-k₄ σ_(s).

If this equality is true, step 225 is followed by step 226.

The gain M is now adjusted.

In the opposite case, step 225 is followed by the test of step 219.

The central unit UC then proceeds to perform a new estimate of thestandard deviation of the distance σ_(2D).

At step 226 the central unit determines the estimated value of thevariance of the phase shift measurement V(φ_(E)) on the basis ofequation (6).

The means 116 of the central unit UC then proceed with step 227 to makea new estimate of the value of the standard deviation σ_(2D) on thebasis of equation: ##EQU26## and to display the result.

It then has to find the value of the weighted mean in accordance withthe invention together with the standard deviation σ_(D) of thedistance.

To do this step 227 is followed by step 228 in which the two parametersW and Z are set to zero.

Block 103 (FIG. 2) is again connected to the converter 101 via theswitch 102 and the means 107 and 108 are connected to the means 105 bythe switch 106.

Then at step 229 the means 103 of the central unit UC verify that thevalue of the signal y_(i) (in dB) is not greater than the upper limit ofthe signal h which can be admitted without distortion.

However, if the value of the signal y_(i) is greater than the limit h,the test 229 is repeated and the samples y_(i) of the correspondingsignal are not accumulated.

In contrast, if the value of the signal y_(i) is not greater than thelimit h the test 229 is followed by step 230 during which the means 105of the central unit test whether the value of the signal y_(i) is lessthan the threshold b.

If it is not less, the test 230 is followed by a return to step 229.

If it is not less than the lower threshold b, the samples Y_(i) areaccumumated and step 230 is followed by steps 231 and 232.

During these steps the means referenced 107 and 108 in FIG. 2respectively determine the signals representative of the parameters:

    Z.sub.i =Z.sub.i-1 +Y.sub.i.sup.2.sub. Δν2        (28)

and

    W.sub.i =W.sub.i-1 +φ.sub.i Y.sub.i.sup.2.sub.Δν2 (29)

Then the means 109 determine the estimated phase variance V(φ_(E)) atstep 233 by generating a signal representative of: ##EQU27##

The means 109 and 111 of the central unit then determine during step 234an estimate of the value of the standard deviation ##EQU28##

In addition, following step 233, the central unit tests at step 235whether the value of the standard deviation σ_(D) less than apredetermined maximum value (σ_(D))_(M) for the standard deviation.

If it is less, then the value of the standard deviation is acceptableand step 235 is followed by step 237.

However, if the value of the standard deviation σ_(D) is greater thanthe predetermined value (σ_(D))_(M), the test 235 is followed by step236. During this step the central unit checks whether the measurementtime t_(m) has not exceeded a predetermined maximum time. If themeasurement time t_(m) is in fact less than the imposed maximum timet_(mM), the test 236 is followed by the above-mentioned step 229 whichis repeated.

However, if the predetermined measurement time has elapsed, the step 236is followed nonetheless by the above-mentioned step 237.

During step 237 the means 111 determine and display firstly the value ofthe distance D, and secondly the value of the standard deviation on thedistance on the basis of equations:

    D=k.sub.5 W.sub.n /Z.sub.n                                 (31)

(based on equation (5)) and ##EQU29## (based on equations (23) and(30)).

It may be observed that it is not essential to know the gain M withaccuracy in order to optimize it, however as mentioned above the valueof the gain M is used to evaluate the average at step 223 and to testthe condition M<M_(M) at step 224. For these reasons, it is desirable toknow the value of M accurately.

Unfortunately, M is a non-linear function of the bias voltage of thephotodiode and also of the ambient temperature. Thus the increments in Mused at step 220 cannot be accurately calibrated.

A fair degree of accuracy on the value of M could be obtained bymodulating the LED 1 so that it is periodically turned off, and then bymeasuring the signal obtained for each passage round the loop in which Mis incremented.

However, it has turned out to be preferable to store a table of valuesin non-volatile or read only memory (130, FIG. 2) giving M as a functionof the bias voltage U (APD) and as a function of temperature θ.

A temperature sensor 131 placed close to the avalanche photodiode 7 thusdelivers an electrical signal to an analog-to-digital converter which isconnected to the data bus of the microprocessor controlling the centralunit UC. Depending on the desired gain "M" requested by themicroprocessor, a digital code is applied to the voltage-controlledpower supply which biases the photodiode.

We claim:
 1. A method of determining the distance between two points comprising the steps consisting in emitting oscillator modulated electromagnetic radiation from a first point towards a second point, in reflecting the said electromagnetic radiation from the second point back towards the first point, in receiving the reflected electromagnetic radiation at the first point by means of a receiver, in comparing the phase of a first signal derived from the receiver and of a second signal derived from the emission modulation, and in generating phase samples φ_(i) representative of the phase difference between the first and second signals, the improvement wherein the method further includes the following steps:(i) in generating a signal representative of the instantaneous variance V(φ_(i)) of the phase samples φ_(i) ; (ii) in generating as an estimation of the phase an intermediate signal derived from the average φ_(E) of n phase samples φ_(i) weighted by their instantaneous variance; (iii) in generating a useful signal representative of the value of the distance D on the basis of the phase estimation φ_(E) ; and (iv) in displaying this distance value.
 2. A method of determining the distance between two points according to claim 1, wherein the the components of the signal detection chain are designed in such a manner that the range of signal amplitude variation passed without distortion by the analog signals covers the width of the distribution function of the received signal amplitudes, i.e. a range of four to six times the standard deviation σ_(s) of the signal.
 3. A method of determining the distance between two points according to claim 1, wherein the equivalent passband Δν₂ of the filters of the signal detection circuits is greater than the turbulance spectrum width ΔF.
 4. A method of determining the distance between two points according to claim 1, wherein the above-mentioned step (ii) is performed by taking as the value representative of the instantaneous variance of the phase samples, a signal which is proportional to the inverse of the square of the value of the noisy signal under consideration over a passband which is slightly larger than or equal to the turbulance spectrum width.
 5. A method of determining the distance between two points according to claim 1, wherein the above mentioned step (i) is performed by means of a detection chain which takes a set of n_(e) samples at a rate n_(e) times greater than the rate at which the n phase samples φ_(i) are taken.
 6. A method of determining the distance between two points according to claim 1, further comprising a preliminary step consisting in controlling an optical attenuator inserted on the propagation path of the electromagnetic radiation in such a manner than the average μ_(s) of the received signal S is less than the upper limit of the signal which can be received without distortion (h) by a factor of k₄ times the standard deviation σ_(s), where k₄ lies in the range 1 to
 2. 7. A method of determining the distance between two points according to claim 1, includes the steps of:(a) measuring the r.m.s. noise value N_(A) ; (b) generating a signal representative of the average μ_(s) and the standard deviation σ_(s) of the signal on the basis of a collection of samples y_(i) of the signal (in decibels) together with the value of the attenuation A_(J) (in decibels) applied to the radiation; and (c) generating a signal representative of an estimate of the phase variance V(φ_(E)) given the r.m.s. noise value, the average σ_(s) and the standard deviation σ_(s).
 8. A method of determining the distance between two points according to claim 7, wherein the method comprises the steps consisting in generating a signal representative of an evaluation of the standard deviation of the distance σ_(1D) proportional to the square root of the phase variance estimation V(φ_(E)), and in displaying the standard deviation.
 9. A method of determining the distance between two points according to claim 8, comprising a further step consisting in increasing the gain M of the receiver in increments until one of the three following conditions is satisfied:(1) receiver bias is optimized; (2) the gain M has reached its maximum permitted value; and (3) the average μ_(s) of the signal has reached an optimum value corresponding to the upper limit of the signal which can be admitted without distortion (h) as reduced by k₄ times the standard deviation σ_(s), where k₄ lies in the range 1 to
 2. 10. A method of determining the distance between two points according to claim 9, wherein the receiver is an avalanche photodiode and the above-mentioned condition (1) is obtained when the noise N_(A) under ambient lighting is about twice the noise N_(O) from the amplifiers.
 11. A method of determining the distance between two points according to claim 9, wherein the generation of the signals representative respectively of the estimation of the phase variance V(φ_(E)) and of the standard deviation of the distance σ_(D) deduced therefrom, together with the display of the standard deviation of the distance is reiterated after adjusting the gain M of the receiver.
 12. A method of determining the distance between two points according to claim 1, wherein steps (i) to (iii) are continued until the standard deviation σ_(D) of the distance is less than a predetermined maximum value (σ_(D))_(M) thus increasing the number n of samples taken and improving the accuracy of the measurement.
 13. A telemeter for performing the method according to claim 1, wherein the telemeter comprises:transmitter means associated with an oscillator suitable for emitting modulated electromagnetic radiation towards an observation point; receiver means suitable for detecting the electromagnetic radiation after reflection at the said observation point; comparator means suitable for comparing the phase of a first signal derived from the receiver and of a second signal derived from the emitted modulation, in order to generate phase samples φ_(i) representative of the phase difference between the first and second signals; the improvement wherein the telemeter further includes: means suitable for generating a signal representative of the value of the instantaneous variance V(φ_(i)) of the phase samples φ_(i) ; processor means suitable for generating an intermediate signal derived from the average φ_(E) of n phase samples φ_(i) weighted by the said value representative of their instantaneous variance;means suitable for generating a useful signal representative of the distance travelled by the electromagnetic radiation between the transmitter means and the receiver means on the base of the said intermediate signal derived from the weighted average φ_(E) ; and display means responsive to the useful signal and adapted to display data representative of the distance travelled by the radiation.
 14. A telemeter according to claim 13, wherein the components of the signal chain are designed in such a manner that the range of signal amplitude variation passed without distortion by the analog signals covers the width of the distribution function of the received signal amplitudes, i.e. a range of four to six times the standard deviation σ_(s) of the signal.
 15. A telemeter according to claim 13, wherein the equivalent passband Δν₂ of the filters of the signal detection circuits is greater than the turbulance spectrum width ΔF.
 16. A telemeter according to claim 13, wherein the means suitable for generating a signal representative of the value of the instantaneous variance comprises means sensitive to the value of the noisy signal under consideration in a passband which is slightly wider than or equal to the turbulance spectrum width.
 17. A telemeter according to claim 13, wherein the processor means are adapted to generate a signal representative of the estimated average of the phase φ_(E) on the basis of the equation: ##EQU30## where φ_(i) represents phase samples and Y_(i)Δν.sbsb.2 represents the value of the noisy signal from a recursive filter having bandwidth Δν₂.
 18. A telemeter according to claim 13, comprising a first detection chain for taking the phase samples φ_(i) and a second detection chain suitable for taking a set of ne samples at a sampling rate which is ne times greater than the rate at which the n phase samples φ_(i) are taken, for the purpose of determining their instantaneous variance V(φ_(i)).
 19. A telemeter according to claim 13, further comprising:an optical attenuator comprising a plurality of filters inserted on the propagation path of the electromagnetic radiation; and control means responsive to the average μ_(s) of the received signal S and suitable for controlling the positioning of the attenuator in such a manner than the said average is kept below the upper limit of the signal which can admitted without distortion by a factor of k₄ times the standard deviation σ_(s), where k₄ lies in the range 1 to
 2. 20. A telemeter according to claim 13, comprising:means for generating a signal representative of the r.m.s. value of the ambient noise N_(A) ; means for generating signals representative of the average μ_(s) and of the standard deviation σ_(s) of the signal expressed dB; means suitable for generating a signal representative of the estimated variance of the phase on the basis of the equation: ##EQU31## in which k₂ and a are constants, t_(m) represents the measurement time, the notation F[ ] represents the normal law distribution function of the centered and reduced variable, and h designates the upper limit of the signal which can be admitted without distortion; and means sensitive to the said signal and suitable for displaying the estimated phase variance.
 21. A telemeter according to claim 20, further comprising: an optical attenuator comprising a plurality of filters inserted on the propagation path of the electromagnetic radiation, and the means suitable for generating signals representative of the average μ_(s) and the standard deviation σ_(s) of the signal expressed in dB are adapted to generate signals which correspond respectively to: ##EQU32## in which equation y_(i) represents the signal samples expressed in dB and A_(j) represents the attenuation of the signal due to the attenuator as associated with respective samples y_(i), while p represents the total number of samples taken into consideration.
 22. A telemeter according to claim 13, including:means for receiving the m noise samples X_(i) and for generating a signal representative of the r.m.s. value of the ambient noise N_(A) such that ##EQU33## means suitable for generating a signal representative of the variance in the estimated phase on the basis of the equation: ##EQU34## k₂ is a constant Y_(i)Δν.sbsb.2 represents the samples of the noisy signal and t_(m) represents the sampling time thereof; and means suitable for displaying information derived from the variance of the estimated phase V(φ_(E)).
 23. A telemeter according to claim 20, further comprising:means suitable for generating a signal representative of an evaluation of the standard deviation of the distance σ_(D) proportional to the square route of the estimation of the phase variance V(φ_(E)), or V(φ_(E)); and means suitable for displaying the standard deviation of the distance σ_(D).
 24. A telemeter according to claim 13, further including:means suitable for controlling incremental increases in the gain M of the receiver until one of the three following conditions is satisfied: (1) the receiver bias is optimized; (2) the gain M has reached its maximum permitted value; and (3) the average of the signal μ_(s) has reached an optimum value corresponding to the upper limit of the signal admitted without distortion (h) as reduced by a factor of k₄ times the standard deviation σ_(s), where k₄ lies in the range 1 to
 2. 25. A telemeter according to claim 24, wherein the receiver is an avalanche photodiode and the above-mentioned condition (1) is obtained when the ambient noise N_(A) is about twice the amplifier noise N_(O).
 26. A telemeter according to claim 23, comprising means suitable for comparing the estimated standard deviation on the distance σ₁ D with a predetermined maximum value (σ_(D))_(M) to control an adjustment of the gain M when the estimated standard deviation σ_(1D) is greater than the maximum value (σ_(D))_(M).
 27. A telemeter according to claim 13, including means suitable for comparing the standard deviation σ_(D) with a predetermined maximum value to control the accumulation of samples and to improve the accuracy of the measurement for as long as the standard deviation is greater than the said predetermined value.
 28. A telemeter according to claim 13, comprising comparator means sensitive to the received signal and adapted to authorize measurement only when the received signal lies between an upper limit h of admissible signal level without distortion and a lower threshold b.
 29. A telemeter according to claim 22, further comprising:means suitable for generating a signal representative of an evaluation of the standard deviation of the distance σ_(D) proportional to the square route of the estimation of the phase variance V(φ_(E)), or V(φ_(E)); and means suitable for displaying the standard deviation of the distance σ_(D).
 30. A telemeter according to claim 29, comprising means suitable for comparing the estimated standard deviation on the distance σ₁ D with a predetermined maximum value (σ_(D))_(M) to control an adjustment of the gain M when the estimated standard deviation σ_(1D) is greater than the maximum value (σ_(D))_(M). 